1. Field of the Invention
This invention relates to an apparatus for controlling the output of a Stirling engine. More particularly, the invention relates to an output control apparatus for a Stirling engine equipped with an automatic transmission.
2. Description of the Prior Art
The output of a Stirling engine, which is an engine of the external combustion type, is determined by the average pressure of a working gas sealed in a working space. When it is desired to raise the output of a Stirling engine, it is necessary to raise the pressure of the working gas in the working space. A typical example of such an output control apparatus for a Stirling engine is disclosed in Japanese Patent Publication (KOKOKU) No. 46-23534. In the disclosed art, however, the feedback of the pressure is performed by piston cylinders and the pressure setting performance is poor. Accordingly, a Stirling engine output control apparatus of a type which relates to the present invention has been proposed to solve this problem. This apparatus will now be described with reference to FIG. 1.
A Stirling engine includes working spaces 1, 2, 3, 4 connected via check valves 5 and a minimum cycle pressure line 6 with a working gas storage tank 7 and a working gas compressor 8. The minimum cycle pressure line 6 has a pressure boosting valve 9. The working spaces 1, 2, 3, 4 are connected via check valves 10 and a maximum cycle pressure line 11 with the compressor 8 and the tank 7, and the maximum cycle pressure line 11 has a pressure reducing valve 12. The opening and closing of the pressure regulating valves 9, 12 of lines 6, 11 is controlled by movement of valve stems 13, 14, respectively. The valve stems 13, 14 are connected to link levers 17, 18 freely pivotable about pivot points 15, 16, respectively. The free ends of the link levers 17, 18 are in contact with a point of application 21 on an operating lever 20 having a grip 19. The operating force acting upon the grip 19 of operating lever 20 can be reduced by suitably selecting the lever ratio of the link levers 17, 18.
The operating lever 20 has a movable pivot point 30 connected to a rack 22 meshing with a pinion 24 fixedly secured to the output shaft of a motor 23. The motor 23 is driven under the control of an electronic control circuit 25. The latter is provided with electric signals from a sensor 26 for sensing displacement of the grip 19 of operating lever 20, a sensor 27 for sensing displacement of the movable pivot point 30 of operating lever 20, a sensor 28 for sensing the line pressure in the minimum cycle pressure line 6, and a sensor 29 for sensing the rotating speed (rpm) of the Stirling engine.
A line 32 having an unload valve 31 is provided for short circuiting the intake and discharge sides of the compressor 8. Since opening the unload valve 31 shorts the intake and discharge sides of the compressor 8, the amount of work done by the compressor 8 is greatly reduced. A solenoid 33 for controlling the opening and closing of the unload valve 31 is actuated by a signal from the electronic control circuit 25.
When the rotating speed of the Stirling engine drops below a fixed value due to application of a load, the unload valve 31 is opened to short circuit the intake and discharge sides of the compressor 8, thereby reducing the amount of work done thereby. Reducing the amount of work prevents a drop in the rotating speed of the Stirling engine to avoid stalling of the engine.
When the driver steps down on the accelerator pedal to displace the grip 19 of operating lever 20 leftward in FIG. 1, the point of application 21 pivots the link lever 17 counter-clockwise about the pivot point 15, whereby the valve stem 13 opens the pressure boosting valve 9. As a result, the working gas in the tank 7 is fed into the working spaces 1, 2, 3, 4 via the minimum cycle pressure line 6 and check valves 5 to elevate the average pressure of the working spaces and, hence, raise engine output. At this time, the electronic control circuit 25 processing the electric signals from the sensors 26, 27, 28, 29 compares a reference target pressure, which depends upon the amount of accelerator pedal depression and the engine rpm, with the actual line pressure. The motor 23 is driven based on the results of the comparison, thereby changing the position of the movable pivot point 30 to control the degree to which the pressure regulating valves 9, 12 open. When the reference target pressure and line pressure become approximately equal, the motor 23 displaces the movable pivot point 30 so as to move the point of application 21 of operating lever 20 to a position that will result in closure of the pressure regulating valves 9, 12. It should be noted that when the driver eases back on the accelerator pedal, the pressure reducing valve 12 is opened by the link lever 18, thereby reducing the pressure in the maximum cycle pressure line 11. The electronic control circuit 25 receiving the outputs of the sensors 26, 27, 28, 29 compares the reference target pressure with the actual line pressure to displace the movable pivot point 30 just as described above.
FIG. 2 is a graph showing the relationship between the reference target pressure and the rotating speed (rpm) of the Stirling engine for accelerator openings of A=0, A=1/4, A=1/2, A=3/4, A=1. When a Stirling engine is equipped with an automatic transmission having parking (P), neutral (N) and drive (D) ranges, certain phenomena occur which now be described with reference to FIG. 3.
Assume that the Stirling engine having an rpm a and a certain reference target pressure while operating in the parking range P or neutral range N is shifted to the drive range D. Since the load will increase at such time, the rpm of the engine is reduced in the manner shown in FIG. 4 until balance (b) is established with the engine output. On the other hand, since the engine rpm necessary for opening the unload valve 31 is c, a state is reached in which b and c become very close together. Since sudden declines in engine rpm occur in the transition from a to b, as shown in FIG. 4, there is a high risk of engine stalling. Stalling of the engine leads to a rise in gas temperature within the engine heater tubes, as a result of which the heater tubes can be damaged.